A number of suitable silicone elastomer materials are available as insulation to provide electrical isolation for the conductors of implantable medical electrical leads, for example, which are employed for cardiac and neurological stimulation and sensing. In many types of implantable leads, the insulation is typically in the form of extruded tubing that has at least one lumen for the passage of electrical conductors therethrough. The insulation tubing is typically required to have a minimum wall thickness (which depends upon the properties of the selected type of silicone elastomer), in order to withstand degradation, for example, due to compressive and tensile loading applied at the time of implant, as well as at explant, and as exerted by the implant environment over the course of implantation, and/or due to abrasion within the implant environment. Some examples of commercially available silicone elastomers which have been, and/or are currently used for the insulation tubing of implantable medical electrical leads include MED-4755, Med-4770 and MED-4719, which are available from NuSil Technology LLC (Carpinteria, Calif.). Silicone elastomers such as these have demonstrated sufficient strength and long term stability/durability to provide the required electrical isolation, for a given wall thickness, when used as the primary insulation in chronically implantable medical electrical leads.
In the past, with the objective to further improve the mechanical properties of silicone insulation tubing, so that the thickness thereof may be reduced, we have investigated the incorporation reinforcing strands/fibers into the wall of silicone insulation tubing, for example, the Dow Corning ETR elastomer, Silastic® 4750, and the NuSil MED-4755, reinforced with either a Polyethylene Terephthalate (PET) polyester fiber or a polyether ether ketone (PEEK) fiber. However, our investigations showed that the inclusion of these types of reinforcing members caused an increase to the wall thickness of the silicone insulation tubing and, thus, to an overall diameter of the corresponding medical electrical lead beyond that which was desired. Such an increase in diameter is contrary to the continued demand for downsized diameters of implantable leads. Thus, there is a need to enhance the properties of silicone insulation for implantable medical electrical leads without increasing the wall thickness thereof, and to even also allow for a reduction in the wall thickness of the silicone insulation, without compromising the mechanical performance and abrasion resistance thereof, on which the integrity of electrical isolation for the resultant lead relies.